<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        body,
        ul,
        ol,
        li,
        img {
            margin: 0;
            padding: 0;
            list-style: none;
        }
        
        #box {
            width: 490px;
            height: 270px;
            padding: 5px;
            position: relative;
            border: 1px solid #ccc;
            margin: 100px auto 0;
        }
        
        .ad {
            width: 490px;
            height: 270px;
            overflow: hidden;
            position: relative;
        }
        
        #box img {
            width: 490px;
        }
        
        .ad ol {
            position: absolute;
            right: 10px;
            bottom: 10px;
        }
        
        .ad ol li {
            width: 20px;
            height: 20px;
            line-height: 20px;
            border: 1px solid #ccc;
            text-align: center;
            background: #fff;
            float: left;
            margin-right: 10px;
            cursor: pointer;
            _display: inline;
        }
        
        .ad ol li.current {
            background: yellow;
        }
        
        .ad ul li {
            float: left;
        }
        
        .ad ul {
            position: absolute;
            top: 0;
            width: 2940px;
        }
        
        .ad ul li.current {
            display: block;
        }
        
        #arr {
            display: none;
        }
        
        #arr span {
            width: 40px;
            height: 40px;
            position: absolute;
            left: 5px;
            top: 50%;
            margin-top: -20px;
            background: #000;
            cursor: pointer;
            line-height: 40px;
            text-align: center;
            font-weight: bold;
            font-family: '黑体';
            font-size: 30px;
            color: #fff;
            opacity: 0.3;
            border: 1px solid #fff;
        }
        
        #arr #right {
            right: 5px;
            left: auto;
        }
    </style>
</head>

<body>
    <div id="box" class="all">
        <div class="ad">
            <ul id="imgs">
                <li><img src="./images/1.jpg" /></li>
                <li><img src="./images/2.jpg" /></li>
                <li><img src="./images/3.jpg" /></li>
                <li><img src="./images/4.jpg" /></li>
                <li><img src="./images/5.jpg" /></li>
            </ul>
        </div>
        <div id="arr"><span id="left">&lt;</span><span id="right">&gt;</span></div>
    </div>
    <script>
    //获取dom对象
    var box = document.getElementById("box");
    var ad = box.children[0];
    var ul = ad.children[0];
    var lis = ul.children;
    var arr = document.getElementById("arr");
    var arrRight = document.getElementById("right");
    var arrLeft = document.getElementById("left");
    var imgWidth = ad.offsetWidth;
    //alert(imgWidth);

    //鼠标经过盒子 显示arr
    box.onmouseover = function () {
        arr.style.display = "block";
    }
    box.onmouseout = function () {
        arr.style.display = "none";
    }

    var pic = 0;
    //点击右侧按钮 移动ul
    arrRight.onclick = function () {
        //如果pic小于最后一张图片的索引 才可以移动ul
        if (pic < lis.length - 1) {
            pic++;
            //target 和 pic有关 和 图片宽度有关 而且是负数
            var target = -pic * imgWidth;
            animate(ul, target);
        }
    }
    arrLeft.onclick = function () {
        //如果pic大于第一张图片的索引 才可以移动ul
        if (pic > 0) {
            pic--;
            //target 和 pic有关 和 图片宽度有关 而且是负数
            var target = -pic * imgWidth;
            animate(ul, target);
        }
    }


    function animate(obj, target) {
        clearInterval(obj.timer);
        obj.timer = setInterval(function () {
            //leader = leader + step
            var leader = obj.offsetLeft;
            var step = 30;
            // //当前位置在目标位置的左侧 所以应该往右走
            // if (leader < target) {
            //     step = step;
            // }
            // //当前位置在目标位置的右侧 所以应该往左走
            // if (leader > target) {
            //     step = -step;
            // }
            step = leader < target ? step : -step;
            //如果对象的位置在目标的左侧 对盒子的位置进行设置
            //如果对象到目标的距离 大于 一步能够迈出的距离
            if (Math.abs(target - leader) > Math.abs(step)) {
                leader = leader + step;
                obj.style.left = leader + "px";
            } else {
                //快要到达目标的时候 手动把对象放置到目标位置上
                obj.style.left = target + "px";
                //到达目标就清理定时器
                clearInterval(obj.timer);
            }
        }, 15)
    }

</script>
</body>

</html>